How do roofers match old slates on a repair?

What has to match

Slates look alike from the ground, but a roofer comparing one against another checks several things. If any is wrong, the repair shows.

• Size: slates are cut to a length and width, and the courses are set out to those measurements, so a different size throws the gauge off and leaves an uneven line.
• Thickness: a thicker slate stands proud and changes the shadow line, while a thinner one may crack under the slate above.
• Colour and texture: natural slate runs from blue-grey to green to near black, and the face may be smooth, rough-split or weathered. Old slates are often paler and duller than new ones.
• Shape: some roofs use shaped slates, such as the pattern on the fishscale slate roof page, which a standard cut cannot match.

The aim is a repair that sits in the roof without drawing the eye. Slate is natural stone and no two pieces are identical, so a perfect match is rare, but a close one is realistic. Comparing a loose slate against the old ones in daylight, and not under a shed light, is how a roofer checks.

Where matching slates come from

A roofer has three main sources, and each has strengths and limits.

The best match is often from the roof itself. A repair can borrow slates from a slope that is out of sight, such as a rear pitch, and replace them with new ones there. The front keeps its weathered look.

Reclaimed stock is the next best, because the stone is already aged. It is covered in is reclaimed slate worth it, and its drawback is the time spent sorting.

Man-made slate is a fourth option, left out of the table. It rarely matches natural slate on an old roof, though a few cases justify it. Whichever source is used, a roofer sorts the slates by size before laying, because even a good batch varies. Slates that are close in thickness but not in length can still be used, in a course set out to suit them.

Making the repair look right

Once the slates are chosen, how they are placed matters as much as what they are. A roofer thinks about the whole slope.

New slates are scattered rather than grouped, so that a patch does not stand out as a lighter block. Where several are needed, mixing them in with existing slates softens the change. On a roof where the front slope is seen from the street, the best slates are kept for the front and the weaker matches go to the back.

The pattern of the courses is kept. Old roofs often diminish in size from eaves to ridge, with the largest slates at the bottom, and a replacement slate has to fit the course it replaces. A slate that is correct in size for a different course will not sit properly.

Fixing counts as well. A repair uses the same lap and the correct fixings for the position, and the roofer will not rely on a bead of sealant. If the nails around the repair are tired, it may be better to fix the new slate with a hook so the old nails are not disturbed. The aim is a repair that lasts as long as the roof around it.

Conservation areas and listed roofs

Matching is a practical matter for most houses and a legal one for some. Thanet District Council's page on conservation areas notes that work which matches the existing is less likely to call for planning permission, which is a reason to keep replacements close to the original.

Dover District Council says an Article 4 direction can reach changes to a house's roof where that part of the roof faces a street, a watercourse or open public land, so check for one before you alter the look of a front slope in the Dover district.

A listed house needs consent under section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 if the work alters its character. A matching slate repair may need no consent in practice, but the council decides, so ask.

• Describe the existing slates and what you propose.
• Send dated photographs of the slopes as they are.
• Keep the council's answer in writing.

The guide to slate roofs in conservation areas covers the wider position, and the slate roofing page the service as a whole.